The present invention relates to imaging method and apparatus using nuclear magnetic resonance (which will be called the "NMR") and, more particularly, to NMR imaging method and apparatus which can completely correct the distortion of projection data generated due to the rise and fall of a gradient field when a signal is detected.
In the prior art, an X-ray CT or an ultrasonic imaging apparatus is widely used as an apparatus for inspecting or imaging in vivo the internal structure such as the head or abdomen of a human body. It is apparent that trials for making similar inspections by the use of the NMR phenomena have succeeded in recent years to obtain data which cannot be attained by the X-ray CT or the ultrasonic imaging apparatus. In the imaging apparatus using the NMR phenomena, the signal from an object to be imaged has to be separated or recognized in accordance with the position.
One of the methods for the necessity provides the data concerning the position by applying a gradient field in a superposed manner to an object to be imaged which is placed in a uniform static field, to make the field intensity different in accordance with the position thereby to make the resonance frequency different at portions. More specifically, the signals of the individual positions are known by applying a gradient field at an instant when the peak of the NMR signal is produced, by observing the process in which the NMR signal is attenuated by the resultant dephasing, and by knowing the frequency components obtained by the Fourier transformation of the observed signal.
In order to obtain the NMR signal, moreover, there is known a method in which, after the nuclear spins are excited by a 90-degree RF pulse, a 180-degree pulse is applied after lapse of a predetermined time .tau. to invert the direction of the nuclear spins thereby to generate an echo. This method has advantages that the signal containing the effect a transverse relaxation time T.sub.2 is obtained by setting the time .tau., and that the unevenness of the static field is compensated. In this method, since the peak of the echo is produced when the time .tau. elapses again from the peak of the 180-degree RF pulse, the gradient field of a desired intensity has to be applied at that instant. In the existing apparatus, incidentally, the gradient field rises at a certain gradient to make it impossible to apply the gradient field of a desired intensity after the above time .tau. has accurately elapsed. In the apparatus of the prior art, therefore, the desired spin echo is not generated by the transient phenomena of the rise of the gradient field to cause the image quality to be degraded.
For example, in the image forming method which is called the "projection-reconstruction method" disclosed by C-M Lai and P. C. Lauterber, Jounal of Physics (E), Scientific Instrument Vol. 13, 1980, pp 747-750, the sampling of the NMR signal is repeated by changing the direction of the gradient field for the signal observation thereby to change the direction of projection. The rotations of such gradient field are effected by changing the magnitudes of the two orthogonal gradient field components and by composing those components. As a result, the influences of the transient response at the start and end of application of the gradient field change with the direction of projection, and there exists in the prior art no effective means for correcting the distortion of the resultant projection data.
Moreover, in the imaging method by the direct Fourier imaging method, as is represented by the spin warp imaging method disclosed by W. A. Edelstein et al., Physics in Medicine & Biology 25, pp 751-756, 1980, gradient field pulses having different magnitudes at individual sampling operations for the phase encoding are applied before the application of the gradient field for the signal observation. The transient phenomena of the rise and fall of that gradient field (i.e., the phase encoding gradient field) affects the linearity between the pulse amplitude and the amount of phase encoding. This results in a problem that the image is distorted.